Q: How to Create EventRelation using REST API I'm trying to create an EventRelation using the REST API, but it does not seem to be working. Here are the details of the request:
URI: /services/data/v37.0/sobjects/EventRelation/
With the following in the body of the request: 
EventId: xxx (ID of the event I want to add a relation to)
RelationId: xxx (ID of a specific contact)
The API doesn't create the EventRelation and just returns the following:
{
  "objectDescribe": {
    "activateable": false,
    "createable": true,
    "custom": false,
    "customSetting": false,
    "deletable": true,
    "deprecatedAndHidden": false,
    "feedEnabled": false,
    "keyPrefix": "0RE",
    "label": "Event Relation",
    "labelPlural": "Event Relations",
    "layoutable": false,
    "mergeable": false,
    "mruEnabled": false,
    "name": "EventRelation",
    "queryable": true,
    "replicateable": true,
    "retrieveable": true,
    "searchable": false,
    "triggerable": false,
    "undeletable": false,
    "updateable": true,
    "urls": {
      "rowTemplate": "/services/data/v37.0/sobjects/EventRelation/{ID}",
      "defaultValues": "/services/data/v37.0/sobjects/EventRelation/defaultValues?recordTypeId&fields",
      "describe": "/services/data/v37.0/sobjects/EventRelation/describe",
      "sobject": "/services/data/v37.0/sobjects/EventRelation"
    }
  },
  "recentItems": [
  ]
}

Is there something I'm doing wrong? Am I not passing required fields? I simply just want to attach a lead or contact to an event.

A: Sounds to me like you just specified GET instead of POST as the method. That returns the describe, instead of trying to create a record. Make sure you set your method to POST.

POST
Notice how the API is clearly trying to create a record here:


GET
Here, the API returns the describe:

